Energy storage devices, including batteries and capacitors, are often used in various electronic devices. Capacitors, for example, are widely used for applications ranging from electrical circuitry and power delivery to voltage regulation and battery replacement. As capacitor technology has continued to develop, several types have emerged. As one example, electric double-layer capacitors (EDLCs), also referred to as ultracapacitors, are characterized by high energy storage and power density, small size, and low weight and have thus become promising candidates for use in several applications.
Ultracapacitors and similar high-surface-area charge storage devices can be used for a variety of purposes, including in microelectronics to store energy, for electrical bypassing in electric circuits, as part of circuitry for power delivery, as a memory storage element, and for a host of other functions. One advantage of ultracapacitors over batteries is that ultracapacitors can be charged and discharged quickly because they do not rely on chemical reactions to store energy, and they don't degrade significantly over their lifetime; even when charged and discharged rapidly. Ultracapacitors are also less sensitive to temperature than are batteries.
Although current ultracapacitors have very high specific power, they have a relatively low specific energy (Wh/kg) compared to battery systems and thus have limited applications for energy storage devices.